You label an angle by either one point and each line or the point at the angle. Take the diagram below. The angle would be labeled as ÐABC or ÐB. The point at the angle is always the middle point.
If two angles have the same measure than they are congruent angles. For example, if ÐA=ÐB then they are congruent.
Two angles are adjacent angles if they share a common vertex and side and have no common interior points.
An interior of an angle is one point that is in-between the two rays that form the angle.
Angle addition postulate
If P is the interior of ÐABC then ÐABP+ÐPBC=ÐABC
Angle classification.
Angles are one of four types. Acute angles are angles below 90 degrees. Obtuse are over 90 degrees. A strait angle is 180 degrees and a right angle is 90 degrees.
Acute Obtuse Right
Strait
Sample problems
If an angle measures 140 degrees than what type of angle is it?
An obtuse angle.
What is the measure of ÐABC if point P is in the interior of ÐABC and ÐPBA=90 degrees and ÐPBC= 20 degrees? 110 degrees
If ÐABC is an obtuse angle, then what is a possible measure of ÐABC?
From above 90 degrees to below 180 degrees.
For you
1 Find ÐHIK
2 What type of angle is ÐABC
3 If you increase ÐABC from the previous question by 40 degrees then what type of angle is it?
4 Find ÐABP if P is the interior of ÐABC, ÐABC=170 degrees and ÐPBC=5 degrees.
If two angles have the same measure than they are congruent angles. For example, if ÐA=ÐB then they are congruent.
Two angles are adjacent angles if they share a common vertex and side and have no common interior points.
An interior of an angle is one point that is in-between the two rays that form the angle.
Angle addition postulate
If P is the interior of ÐABC then ÐABP+ÐPBC=ÐABC
Angle classification.
Angles are one of four types. Acute angles are angles below 90 degrees. Obtuse are over 90 degrees. A strait angle is 180 degrees and a right angle is 90 degrees.
Acute Obtuse Right
Strait
Sample problems
If an angle measures 140 degrees than what type of angle is it?
An obtuse angle.
What is the measure of ÐABC if point P is in the interior of ÐABC and ÐPBA=90 degrees and ÐPBC= 20 degrees?
110 degrees
If ÐABC is an obtuse angle, then what is a possible measure of ÐABC?
From above 90 degrees to below 180 degrees.
For you
1 Find ÐHIK
2 What type of angle is ÐABC
3 If you increase ÐABC from the previous question by 40 degrees then what type of angle is it?
4 Find ÐABP if P is the interior of ÐABC, ÐABC=170 degrees and ÐPBC=5 degrees.
Helping sites
http://library.thinkquest.org/2647/geometry/angle/measure.htm
http://www.mathsisfun.com/angles.html
http://www.mathsisfun.com/geometry/congruent-angles.html